As textile substrates age, their color tends to fade or yellow due to exposure to light, air, soil, and natural degradation of the fibers that comprise the substrates. As such, to visually enhance these textile substrates and counteract the fading and yellowing the use of polymeric colorants for coloring consumer products has become well known in the prior art. For example, it is well known to use whitening agents, either optical brighteners or bluing agents, in textile applications. However, traditional whitening agents tend to lose efficacy upon storage due to deleterious interactions with other formulation components (such as, for example, perfumes). Further, such whitening agents can suffer from poor deposition on textile substrates. As such, formulators tend to increase the level of whitening agent used to counteract any efficacy lost upon storage and/or to increase the amount of whitening agent available to deposit on the textile substrate.
Leuco dyes are also known in the prior art to exhibit a change from a colorless or slightly colored state to a colored state upon exposure to specific chemical or physical triggers. The change in coloration that occurs is typically visually perceptible to the human eye. Many of these compounds have some absorbance in the visible light region (400-750 nm), and thus more or less have some color. In this invention, a dye is considered as a “leuco dye” if it did not render a significant color at its application concentration and conditions, but renders a significant color in its triggered form. The color change upon triggering stems from the change of the molar attenuation coefficient (also known as molar extinction coefficient, molar absorption coefficient, and/or molar absorptivity in some literatures) of the leuco dye molecule in the 400-750 nm range, preferably in the 500-650 nm range, and most preferably in the 530-620 nm range. The increase of the molar attenuation coefficient of a leuco dye before and after the triggering should be greater than 50%, more preferably greater than 200%, and most preferable greater than 500%.
Leuco compounds can be used as whitening agents in laundry care compositions (e.g., laundry detergents). In such uses, the addition of the leuco compound, which is an uncolored or only lightly colored state, does not significantly affect the aesthetics of the laundry care composition. Then, the leuco compound can be converted to a colored state in which it imparts the desired whitening benefit to the textile substrate. Leuco compounds are effective for this use to the extent that they maintain a colorless form on storage in a laundry care composition (e.g., a detergent) and undergo a triggered change to a colored or much more highly colored state during or after use by the consumer. The challenge is to keep the leuco in its uncolored form over potentially long storage periods, yet have the leuco converted through triggering mechanism upon use by the consumer. In some cases, the leuco compounds may be stabilized through an addition of separate antioxidant compounds. However, because of the variation in the structure of both leuco compounds and the antioxidants, there is no expectation that they will necessary be co-located in any formulation comprising both. There is thus a continuing need to address the ability to stabilize the leuco form in any composition in which it may be used.
We have addressed this problem by covalently attaching a leuco moiety to an antioxidant moiety to ensure their colocation in any formulations in which this compound is used. We have discovered that leuco moieties conjugated (covalently bound) to antioxidant moieties (e.g., hindered phenols) deposit through the wash to provide a whiteness benefit, but surprisingly these leuco conjugates (leuco moiety covalently attached to antioxidants) impart greater storage stability over their parent leuco compound even when accompanied by an equimolar amount of antioxidant compound.